There is an ever increasing use of microwave frequencies for both commercial and military environments and, accordingly, there are great demands for high accuracy frequency measuring devices. In particular, there are demands for frequency measuring devices having extremely broad bandwidth capabilities, and good FM tolerance, sensitivity, and acquisition time. Specifically, one of the demands that is now being made is for commercial frequency measuring equipment operable in the 10 Hz to 18 GHz range.
One of the difficulties that has been encountered with respect to known devices is their relatively long acquisition time. While some devices have been able to improve sensitivity, they have done so at the expense of decreased FM tolerance. Of more pressing concern, however, is the fact that even "state-of-the-art" solid state devices that can be used for frequency measurement are incapable of switching in the upper spectrum of the microwave frequency range noted above. That is, many direct counting digital frequency counters, which may be considered "state-of-the-art" or "off-the-shelf", are incapable of direct counting at frequencies above 1000 MHz. Thus, for any frequency measuring device to operate in ranges over 1000 MHz, it is necessary to provide circuits which convert or prescale the microwave frequencies down to a frequency that can be counted by such direct counting digital frequency counters.
Several techniques have been used by prior art frequency counters and frequency meters to pre-condition microwave frequencies down to countable levels. Two of the most popular are the transfer oscillator technique and the heterodyne technique. In a transfer oscillator type of frequency counter, a low frequency (usually 100 to 200 MHz) oscillator is phase-locked to a subharmonic of the signal to be measured. Circuit means are provided to determine the harmonic number of the sub-harmonic; and the output of the transfer oscillator -- the low frequency oscillator -- is multiplied by the harmonic number to provide an indication of the unknown frequency being measured. This technique can be carried out by using direct counting digital circuitry with a gate time controlled by a factor equivalent to the harmonic number which has been determined, so that the resulting number counted by the direct counting circuitry is equivalent to the microwave frequency.
Although the transfer oscillator technique for measuring high frequency signals, especially signals in the microwave frequency range, provides good sensitivity, because of the relatively narrow bandwidth of the phase-locked loop used, transfer oscillator circuits employing this technique have a number of disadvantages, especially when used in the upper (GHz) frequency ranges. These disadvantages include:
a. Long acquisition time. [For a counting resolution to 1 Hz, a time period equal to the number of seconds multiplied by the harmonic number, is required. The long acquisition time also results from the fact that a phase-locked loop has an appreciable finite lock-in time, and resolution cannot begin until phase-lock is established.]
b. Poor FM tolerance. [This is a result of the limited bandwidth of the phase-locked loop.]
c. In a short measurement interval, poor resolution of the frequency being measured. [This is a corollary of (a) above.]
An improved transfer oscillator having higher FM tolerance is taught in Voyles et al, U.S. Pat. No. 3,781,678 issued Dec. 25, 1973. In that patent there is taught an automatic transfer oscillator including a frequency locked loop in combination with harmonic number determination circuitry. The counting circuit counts both the frequency of a local oscillator signal and the intermediate frequency produced by the frequency locked loop concurrently over a given time period.
Because of the disadvantages of transfer oscillators, discussed above, a more frequent commercial embodiment of automatic frequency counters, capable of counting into the microwave frequency range, utilizes a heterodyne circuit. Typically, a heterodyne counter converts down the frequency being measured to directly countable frequencies using a mixer. The signal being measured is fed to one input port of the mixer and a local oscillator signal is fed to another input port of the mixer. A low pass filter passes the difference frequency (between the input signal and the local oscillator signal) output of the mixer. If no difference frequency -- referred to as an intermediate frequency or I.F. -- is sensed with the local oscillator set at its lowest frequency, the local oscillator is stepped by an integral amount, and the presence or absence of an I.F. at the output of the low pass filter is again looked for by a detector. If none is noted, the local oscillator frequency is again stepped. This sequential stepping continues until such time as an I.F. is detected. The low pass filter's bandwidth is such that any I.F. frequency detected is within the direct counting capability of a digital counter. The I.F. detected is counted and the count is added to the local oscillator frequency to determine the frequency of the input signal and the result is displayed on a digital display. In such arrangements the local oscillator frequency must always be below the frequency of the input signal, so that the difference frequency or I.F. may be added to the local oscillator frequency, thereby requiring that every measurement sequence start at the lowest oscillator frequency and step upwards until such time as a countable I.F. is sensed.
The advantages of the use of a heterodyne technique in the measurement of microwave frequency signals include good FM tolerance and a resolution time no greater than one second, to obtain 1 Hz resolution. However several disadvantages exist, particularly when this technique is used for the measurement of the frequency of signals in the microwave frequency range. Among these disadvantages are the following:
a. Acquisition time is longer than desirable when the frequency of the signal to be measured is high. [Because the local oscillator frequency must be below the frequency of the signal to be measured, it is necessary to always start at zero or a low level and step the local oscillator until such time as an I.F. is sensed and passed to the counter.]
b. High frequency signal sensitivity is poor. [The local oscillator signal occurs at the output of a harmonic generator, the particular step harmonic being selected by a harmonic selection filter. As the frequency increases, the power available decreases, whereby mixer losses decrease sensitivity.]
In addition, the tuneable (harmonic selection) filter of a heterodyne frequency counter must have its pass bands very tightly controlled, so that only the selected harmonic is present at the local oscillator input to the mixer. This disadvantage will be more readily understood if it is realized that, if the tuneable or harmonic selection filter allows any energy from any unwanted harmonic into the mixer, two adjacent harmonics may beat together in the mixer, causing the production of a spurious signal, which may be passed by the low pass filter so as to mask the desired I.F. signal. Or, at high frequencies, the wrong harmonic may be chosen resulting in an erroneous display based on the addition of the I.F. to the wrong local oscillator frequency, whereas the actual frequency may be one or more orders of magnitude, (of the harmonic generator base frequency) away from the displayed frequency.
As noted above, one disadvantage of heterodyne frequency counters is the decreased power that is available from the local oscillator at higher frequencies, and the resultant decrease in sensitivity. The conversion losses in the mixer may reach such a level that no useable I.F. can be detected. Obviously, in order to minimize this problem, the base frequency of the harmonic generator -- the reference frequency -- should be as high as possible. In order to accommodate a higher reference frequency, however, a faster direct count capability is required, as well as a wider I.F. bandwidth. Concomitant with faster direct counting capabilities and wider I.F. bandwidth, however, is increased noise in the I.F. circuitry, and a resultant decrease in the sensitivity of the counter.
Several prior art heterodyne-type frequency counters are described in: U.S. Pat. No. 3,403,338 issued to Martin on Sept. 24, 1968; U.S. Pat. No. 3,750,014 issued to Gaw on July 31, 1973; l and U.S. Pat. No. 3,662,261 issued to Barthold et al on May 9, 1972.
In order to increase the sensitivity and acquisition time of prior art heterodyne-type frequency counters, sophisticated tuned filters using YIG (Yttrium-iron-garnet) spheres have been developed. For example, the Barthold et al patent teaches a device where a YIG member is placed in a carefully machined slot in the center conductor of a coaxial line. Obviously, the expensiveness of these filters makes frequency counters using them more expensive than desirable.
In any event, it will be apparent to those skilled in the art that an improved microwave frequency counter, (e.g. a frequency counter having an extremely broad frequency counting range) can be obtained if wider harmonic spacing can be utilized (with a resultant decrease in the requirement for the inclusion of a critical, controllable or tuneable filter) and if the reference oscillator frequency, upon which the harmonics of an harmonic generator are based, is higher than the maximum direct counting capability of a digital counter included in the system.
Therefore, it is an object of this invention to provide a frequency counter for measuring the frequency of an input signal thereto, over a very broad frequency bandwidth, and providing a digital display of the frequency being measured to any desired resolution in a short period of time, e.g., one second for 1 Hz resolution.
Another object of this invention is to provide a frequency measurement system wherein the frequency of the input signal may be many times higher than the direct count capability of a digital counter included in the system.
A further object of this invention is to provide a frequency counting method and apparatus having high sensitivity and FM tolerance, and relatively short acquisition time. Yet a further object of this invention is to provide a new and improved frequency counter wherein all of the measuring components operate at lower frequencies then the microwave frequencies normally measured.
Another object of this invention is to provide a very wide bandwidth microwave frequency counter wherein the components used in the circuits of the counter, notwithstanding its very wide bandwidth, are relatively inexpensive and non-critical in design.
A still further object of this invention is to provide an improved method of making frequency measurements of input signals whose frequency is unknown, but may be in the microwave range.